Environmental testers, such as thermostat chambers and thermostat-humidistat chambers used for material tests, which can stably keep physical quantities, such as temperature, humidity, etc., in a test chamber as closed space to predetermined target values have been known (see, e.g., Patent Document 1). In such an environmental tester, a temperature sensor and a humidity sensor are provided in the test chamber surrounded by adiabatic walls, and an air conditioner including a refrigerator, a humidifier, and a heater is controlled based on the values measured by the sensors. Thus, air is circulated between the test chamber and the air conditioner, and the temperature and the humidity in the test chamber are kept to target temperature and target humidity, respectively.